Television channels broadcast signals on Very High Frequency (VHF) and Ultra High Frequency (UHF) bands. In order to prevent interference between adjacent channels within these frequency bands, white spaces are created which are areas where no television signals will be transmitted. On Nov. 4, 2008, the Federal Communications Commission (FCC) of America licensed the use of white spaces for signal transmission by private individuals. The recent switch from analog television to digital has further freed up large amounts of space on the Television White Space (TVWS) spectrum, with the FCC stopping analog transmissions on Jun. 12, 2009. Current regulations in North America allow the use of signals transmitted from portable devices on the frequency band occupied by television channels 21˜51, with the proviso that these signals must not interfere with transmissions from television stations and wireless microphones.
The three main issues confronting the use of the TVWS band for signal transmission are: spatial variation; spectrum fragmentation; and temporal variation. Spatial variation is due to the fact that the availability of channels on the TVWS spectrum depends on location. It cannot be assumed that a particular channel which is free in a first location is also free in a second location. Temporal variation means that a channel may only be free for a limited amount of time. Just as it cannot be assumed that a same channel is free in a first location and a second location, it cannot be assumed that a same channel is free at a first time and a second time. If the channel becomes unavailable, reconfiguration must be performed in order to continue transmissions. Finally, spectrum fragmentation refers to the issue wherein different regulatory bodies use different bandwidths for TVWS band operation. For example, in the United States, Taiwan, South Korea and Japan, individual channels occupy a bandwidth of 6 MHz; in Italy, Australia and New Zealand, individual channels occupy a bandwidth of 7 MHz and in the United Kingdom, France and China, individual channels occupy a bandwidth of 8 MHz. These issues mean that it is necessary to have access to information detailing which channels are currently available, for how long the channels are available, and bandwidth information for the particular region, before signal transmission can begin.
Many devices which operate on the TVWS band do not have the ability to determine their own location information and therefore cannot select a particular bandwidth for scanning. This means that they must scan on all bandwidths in order to locate access point (AP) beacons. After a first AP beacon is received, the corresponding bandwidth of the beacon is used to determine in which area the GDC dependent STA is located. This method is inefficient and can take up a considerable amount of time and resources.